1. Oil Shale
A particularly attractive alternative source of energy is oil shale, the attractiveness stemming primarily from the fact that oil can be “extracted” from the shale and subsequently refined in a manner much like that of crude oil. Technologies involving the extraction, however, must be further developed before oil shale becomes a commercially-viable source of energy. See J. T. Bartis et al. Oil Shale Development in the United States: Prospects and Policy Issues, RAND Corporation, Arlington, Va., 2005.
The largest known deposits of oil shale are found in the Green River Formation, which covers portions of Colorado, Utah, and Wyoming. Estimates on the amount of recoverable oil from the Green River Formation deposits are as high as 1.1 trillion barrels of oil—almost four times the proven oil reserves of Saudi Arabia. At current consumption levels (˜20 million barrels per day), these shale deposits could meet the energy demands of the United States for the next 140 years (Bartis et al.).
Oil shale typically consists of an inorganic component (primarily carbonaceous material, i.e., a carbonate) and an organic component (kerogen). Thermal treatment can be employed to break (i.e., “crack”) the kerogen into smaller hydrocarbon chains or fragments, which are gas or liquids under retort conditions, and facilitate separation from the inorganic material. This thermal treatment of the kerogen is also known as “thermal upgrading” or “retorting,” and can be done at either the surface or in situ, where in the latter case, the fluids so formed are subsequently transported to the surface.
In some applications of surface retorting, the oil shale is first mined or excavated, and once at the surface, the oil shale is crushed and then heated (retorted) to complete the process of transforming the oil shale to a crude oil—sometimes referred to as “shale oil.” See, e.g., Shuman et al., U.S. Pat. No. 3,489,672. The crude oil is then shipped off to a refinery where it typically requires additional processing steps (beyond that of traditional crude oil) prior to making finished products such as gasoline, lubricant, etc. Various chemical upgrading treatments can also be performed on the shale prior to the retorting. See, e.g., So et al., U.S. patent application Ser. No. 5,091,076.
2. In Situ Upgrading of Shale-Bound Kerogen
A method for in situ retorting of carbonaceous deposits such as oil shale has been described in U.S. Pat. No. 4,162,808. In this method, shale is retorted in a series of rubblized in situ retorts using combustion (in air) of carbonaceous material as a source of heat.
The Shell Oil Company has been developing new methods that use electrical heating for the in situ upgrading of subsurface hydrocarbons, primarily in subsurface formations located approximately 200 miles (320 km) west of Denver, Colo. See, e.g., U.S. Pat. Nos. 7,121,342 and 6,991,032. In such methods, a heating element is lowered into a well and allowed to heat the kerogen over a period of approximately four years, slowly converting (upgrading) it into oils and gases, which are then pumped to the surface. To obtain even heating, 15 to 25 heating holes could be drilled per acre. Additionally, a ground-freezing technology to establish an underground barrier around the perimeter of the extraction zone is also envisioned to prevent groundwater from entering and the retorting products from leaving. While the establishment of “freeze walls” is an accepted practice in civil engineering, its application to oil shale recovery still has unknown environmental impacts. Additionally, the Shell approach is recognized as an energy intensive process and requires a long timeframe to establish production from the oil shale.
3. Chemical Upgrading of Shale-Bound Kerogen
Chemical routes to the upgrading of shale-bound kerogen have many advantages—particularly with regard to in situ upgrading. Such molecular-based methodologies generally entail contacting the kerogen with a reactive species capable of breaking carbon-carbon bonds within the kerogen and/or bonds between the kerogen and inorganic components of the shale. A result of such bond breaking is a more mobile kerogen-derived molecule that can more easily be transported out of the subsurface formation in which it was formed. Such methodologies are attractive for a variety of reasons including, but not limited to, lower energy requirements, scalability, specificity and flexibility. Such methodologies have been described in U.S. patent application Publication 2008/0006410 A1. Once at the surface, the kerogen based product can be further processed.
A need remains for efficient and effective methods for chemically upgrading shale-bound kerogen and extracting kerogen from oil shale deposits in order to better take advantage of oil shale as an alternative source of energy.